Electrophotography has recently come to be extensively used in and applied to electrostatic copiers, facsimile, laser beam printers, etc. owing to the advantages thereof including instantaneousness and the ability to give high-quality images. The electrophotographic photoreceptors used in these image forming apparatus mainly are so-called organic photoreceptors obtained by forming a photosensitive layer including a charge-generating agent, a charge-transporting agent, and a binder resin on a conductive support.
However, the electrophotographic photoreceptor obtained by directly coating a conductive support with a photosensitive layer has a possibility that since the conductive support is close to the photosensitive layer, charges might be injected into the photosensitive layer. There are hence cases where microscopic disappearance of surface charges or a microscopic decrease in the amount of surface charges occurs, resulting in image defects.
In addition, formation of a photosensitive layer having an even thickness is difficult because of the influence of the surface state of the conductive support. The resultant unevenness in the thickness of the photosensitive layer may cause image defects such as density unevenness and pin-holes. Such images are formed especially in high-temperature and high-humidity atmospheres.
A technique for preventing such image defects is being employed in which an undercoat layer is disposed between the conductive support and a charge generation layer, for example, in order to prevent charge injection from the conductive support, to conceal surface defects of the conductive support, and to improve adhesion between the photosensitive layer and the support. For the undercoat layer, an organic-solvent-soluble polyamide resin or the like is used (see, for example, patent documents 1 to 9).
Meanwhile, electrophotographic photoreceptors having a single undercoat layer constituted of a conventional polyamide resin or the like undergo considerable accumulation of residual potential, and there are cases where a considerable decrease in sensitivity, image fogging, and the like come to occur with the lapse of time.
For the purposes of mitigating the residual-potential accumulation due to the influence of the conductive support and preventing image defects, a technique is being employed in which an undercoat layer constituted of an organic-solvent-soluble polyamide resin which contains fine particles of a metal oxide is disposed on the conductive support (see, for example, patent documents 4 to 9).
Furthermore, a technique in which an undercoat layer or an interlayer is laminated on a conductive support and a technique in which an N-alkoxy(methoxy) methylated nylon is incorporated into an undercoat layer or interlayer are being employed, and are regarded as effective means for inhibiting charge injection from the conductive support and enhancing the effect of inhibiting background soils (see, for example, patent documents 8 and 9).
Meanwhile, electrophotographic photoreceptors employing an organic photoconductive substance have various advantages. However, these electrophotographic photoreceptors do not satisfy all the properties which are required of an electrophotographic photoreceptor. In particular, when such an electrophotographic photoreceptor is repeatedly used in a copier or a printer, the photosensitive layer deteriorates gradually. There is hence a desire for an electrophotographic photoreceptor which suffers little damage by repeated use, has high sensitivity and a low residual potential, and retains stable electrical properties.
These properties depend considerably on the charge generation substance, charge transport substance, additives, and binder resin.
Phthalocyanine pigments and azo pigments are mainly used as the charge generation substance, since the charge generation substance must have sensitivity to the light source for light input. Although various kinds of substances are known as the charge transport substance, amine compounds among these show an exceedingly low residual potential and are hence being utilized extensively (see, for example, patent documents 10 and 11).
As described above, a large number of photoreceptor materials including charge generation substances, charge transport substances, and binder resins are known. However, even when materials known to have high performance are selected from these at random and used in combination, this does not make it possible to provide an electrophotographic photoreceptor which has excellent electrophotographic photoreceptor properties and which, when used in an image forming apparatus, actually gives desired images of high quality.
Especially in recent years, an improvement in wear resistance is desired. One means for meeting the desire is a technique in which a binder resin having excellent wear resistance is used in a charge transport layer and the content of the charge transport substance is reduced to thereby minimize the decrease in the performance of the binder resin.